With the progress of our present-day electronic society, electromagnetic noise pollution is becoming a new source of industrial pollution. Such electromagnetic noise can be divided into two types. One type consists of conduction from power supply lines connected directly to digital equipment, and the like, while the other type consists of the radiation of spontaneously generated electromagnetic wave leakage from digital equipment and the like. In both cases, such electromagnetic noise has a deleterious effect on other digital equipment, and may lead to the malfunctioning of such equipment. In cases where preventive measures are required, noise can be prevented relatively simply by means of various types of noise filters. In the case of radiated noise, various preventive measures have been used, both in digital equipment which generates electromagnetic wave leakage and digital equipment which is affected by such leakage, but in many cases the prevention of such noise is rather difficult. In order to prevent such radiation noise, it is first necessary to accurately measure the electromagnetic wave leakage from the digital equipment involved.
Conventionally, electromagnetic wave leakage from digital equipment has been measured by various methods. In one method the digital equipment being measured is placed inside a large shielded room along with a measuring instrument such as an antenna, and a measurer, after which power is supplied to the digital equipment being measured and the intensity of the radiation field created by the actually occuring electromagnetic wave leakage is measured (as in the measurement method described in CISPR Standards-Publ. 22. In another a portion of a high-frequency transmission line used for connection to the digital equipment in question is placed inside a brass tube of a fixed shape, which acts as an antenna, and the power value of the electromagnetic wave leakage occuring during signal transmission is measured (as in the measurement method for measuring electromagnetic wave leakage using high-frequency transmission lines described in MIL Standard MIL-T-81490). Other methods used include the absorption clamp method described in CISPR Standards-Publ. 16, the surface transfer impedance method described in MIL Standard MIL-C-85485, the electric field method described in NASDA Standard NASDA-QTS-1012, and the cross talk measurement method described in VDE Standard VG95373 Part 15. In such cases, the nature and frequency band of the signal involved are ascertained and the measurement method used is selected so that it will be the most suitable method for the packaging conditions involved.
However, in the case of the measurement method described in CISPR Standards-Publ. 22, measurements cannot be performed without the provision of an expensive shielded room which occupies a large area. Furthermore, the measurement method described in MIL Standard MIL-T-81490 is a measuring method for high-frequency transmission lines and cannot be used to perform measurements in the case of general digital equipment. Further, this method is incapable of frequency sweeping. In the case of the remaining measurement methods described in public standards, the effects of quasi-electrostatic fields and induction fields cannot be eliminated. Accordingly, electromagnetic wave leakage is compared relatively to the power value of electromagnetic wave leakage from other electromagnetic equipment used as a standard. Since an absolute power value cannot be obtained, comparison is difficult. Then, there is no single measurement method which always satisfies all requirements.